Certain ethnic minorities may be more prone to such problems, adds Thomson – not least because many minorities are often at greater risk of poverty to begin with. But take people of a Black African or Black Caribbean background, who are much more likely to have sickle cell disease. This term refers to multiple conditions in which people's red blood cells are unusually shaped. Sometimes that leads to painful episodes called sickle cell incidents or crises.

"Cold is a trigger for a sickle cell incident," says Thomson. A 2015 review found that it was "very real" for people with sickle cell disease, was widely reported by patients and also mentioned in textbooks. However, the authors added that there was "surprisingly little" scientific evidence to explain exactly how temperature or temperature changes might affect sickle cell incidents.